Check our
www.fpi-protostar.com/bgreer/ and the topics discussing fans and thermal stability in newts.
See also the yahoo group skyquest_telescopes Folders and Photos sections for some discussions on how some folks modify their scope to address mirror cooling and the issue of boundary layer.
A nice trick is to select a bright star or planet, and defocus until you have as large a bright disc as possible. Look at the bright disk - study it for a few minutes - and you will see one or two types of phenomenon. One is a slow churning type of distortion without any directional pattern. These are thermals due to your mirror not being at ambient. If you turn on a fan at this point, and the system is designed to somehow move air across the front of the scope to "scrub" this boundary layer, you'll see this slow churning distortion resolve or improve. If your mirror is perfectly equilibrated with ambient temperature you will not see these mirror thermals. Or, if it isn't but you have a very effective boundary layer scrubbing fan system, you also won't see it or it will be much less noticeable.
If the mirror is at ambient, or even if it is not and you study the image, you'll see a second type of distortion: An extrememely fast almost eyeblink quick distortion that seems to sweep straight across the image. This is atmospheric seeing effects. Cooling your mirror will have no effect on this, obviously.
Lotsa opinions on impact of fans on image quality in newts, very little hard data, but greer's page above has some of it, and Alan Adler had an article in S&T also about this.
http://www.pha.jhu.edu/~atolea/WAS/t...newtonians.pdf
You'll not notice undetectable (edited typo from original post) impact on low contrast low resolution objects like galaxies and nebula. That's why DSO viewers will often dismiss the value of fans. You will notice the most impact from mirror thermals on bright high contrast objects like Jup, Sat, and the Moon at high power. I think this is one reason why newts traditionally have a bad rep for such objects, compared to refractors..that and the propensity for poor collimation to show bad images of planets and moon, which isn't an issue for refractors.
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